Paper pulps called “mechanical pulps” or “high-yield pulps” or “wood pulps” are obtained directly from wood by a sequence of mechanical treatments, generally referred to collectively as mechanical “refining”, and carried out by means of grindstones and/or refiners. The mechanical paper pulp is subsequently subjected to a bleaching phase, which may comprise one or more stages, depending on the degree of whiteness required.
The advantage of a mechanical pulp production process is its high material yield as compared with a “chemical” pulp production method. The reason is that, unlike the chemical pulp production methods, in which the lignin present in the untreated wood is removed almost entirely by cooking in the presence of chemical products, around 90% of the untreated wood is conserved in the pulps obtained at the outcome of a mechanical pulp production method.
The mechanical refining in a mechanical pulp production method typically comprises a number of refining steps, such as a primary refining operation, generally called “defibering”, a secondary refining operation, a tertiary refining operation, an operation of refining screening wastes, etc. These refining steps provide pulps which have different degrees of refining, in order to progressively transform the wood into individualized fibers and so to allow the production of paper pulp.
Mechanical refining has the drawback of being highly energy-consuming, consuming typically from 1500 to 3000 kWh per metric ton of mechanical pulp produced. This energy on the one hand represents a substantial cost and on the other hand may cause damage to the wood fibers. Various pathways have therefore been conceived in order to reduce the required energy.
Accordingly, document EP 1728917 proposes carrying out a refining operation at low consistency, in other words at low pulp dry matter content. A treatment of this kind, however, requires a host of apparatus, and is of limited efficacy.
Document WO 08081078, in turn, proposes a mechanical pulp production method comprising a step of ozone treatment during refining. This treatment, however, has the drawback of giving rise to chromophoric groups on the polysaccharide molecules contained in the wood, these groups proving difficult to oxidize when the pulp is subsequently bleached conventionally.
Moreover, a particular interest has developed in enzymatic wood treatments within mechanical pulp production methods, owing to their gentle environmental impact.
Known accordingly is document U.S. Pat. No. 6,267,841, which describes a mechanical pulp production method comprising an enzymatic treatment step performed between two refining steps or prior to one refining step. The enzyme is selected from pectinases, xylanases, laccases, cellulases, manganese peroxidases, and mixtures thereof. These treatments, however, have the drawback of degrading the wood fibers, and/or require refining to be carried out at a high temperature, thereby limiting the energy saving that is realizable.
Documents EP 429422 and WO 91/11552 also describe mechanical pulp production methods which comprise a step of enzymatic pretreatment of a fibrous material for the purpose of facilitating its subsequent refining. In document EP 429422, the redox potential of the enzymes described is adjusted using regulators such as gaseous oxygen or nitrogen, antioxidants, sugars, organic acids, or inorganic salts. In document WO 91/11552, a recommendation is made to carry out the enzymatic pretreatment beyond a certain redox potential. Adjusting the redox potential of the enzymes, however, is a delicate operation, and proves to be costly.
Document EP 0745 154 describes a chemical pulp production method employing a multiple-component system for the modification, decomposition, or decoloring of the lignin, comprising in particular an oxidoreductase enzyme, a mediator, a free amine and an oxidizing agent. This system is employed for bleaching a chemical pulp which has been delignified with oxygen beforehand. This system has the drawbacks of generating effluents that are harmful to the environment, and of giving rise to high production costs.
Optimizing the enzymatic activity of laccases, moreover, has been studied in document U.S. 2008/0189871. This document proposes an LMS system (Laccase Mediator System) comprising a mediator derived from 2,6-dimethoxyphenol. This system is employed for bleaching a cloth. It is stated on the one hand that it may be used during the manufacture of pulp and on the other hand that it may be used during the bleaching of a pulp.
The methods and the products used in the prior art, therefore, do not provide complete satisfaction.
In particular there is still a need existing to reduce the energy demand of mechanical pulp production methods and to ensure, furthermore, a mechanical pulp having papermaking qualities that are equivalent to or an improvement on those obtained by the known techniques. There is also a need existing for a mechanical pulp having a degree of whiteness greater at the outcome of refining, and/or developing an improved capacity for bleaching, relative to those obtained by the known techniques. A need exists, lastly, to reduce the amount of chemical products required to bleach a mechanical pulp while ensuring an equivalent or improved degree of whiteness in relation to the mechanical pulps obtained with the techniques of the prior art.